1. Field of the Invention
The present invention relates to an optical transmitter-receiver module used in an optimal communication system, and more particularly to an optical transmitter-receiver module which has an integration of a transmitter circuit and a receiver circuit with a reduced crosstalk between the transmitter circuit and the receiver circuit.
2. Description of the Related Art
The optical transmitter-receiver module is suitably applicable to various data communication systems, typically, local area networks and wide area networks. The optical transmitter-receiver module has requirements for further improvement in high speed performance, further cost reduction, and further side reduction or shrinkage. For satisfying those requirements, it is essential that an optical transmitter-receiver device is packaged in compact over a single substrate, wherein the optical transmitter-receiver device has an integration of the optical transmitter circuit and the optical receiver circuit. The optical transmitter circuit includes light emitting devices, whilst the optical receiver circuit includes light receiving devices. In general, a minimum output current from the light receiving device is much smaller than a driving current for driving the light emitting device. For example, the driving current for driving the light emitting device is 100 mA, whilst the minimum output current from the light receiving device is 10 micro-A, so that a difference is 80 dB.
The optical transmitter-receiver device often uses a standard connector MT-RJ. In this case, a distance between the light emitting device and the light receiving device should be narrow, for example, 750 micrometers. If the optical transmitter-receiver device is required to perform a high bit rate, for example, 10 Gbps or higher, then an undesirable crosstalk between the transmitter circuit and the receiver circuit becomes remarkable.
In order to reduce the crosstalk between the transmitter circuit and the receiver circuit, it is effective to provide a shielding plate between the transmitter circuit and the receiver circuit. This idea is disclosed in 2000 Electronics Information Communication Society SC-3-7, entitled xe2x80x9cAnalysis of Crosstalk for MT-RJ Optical Sub-Assemblyxe2x80x9d.
FIG. 1 is a schematic perspective view illustrative of a conventional optical transmitter-receiver module having an integration of the optical transmitter circuit and the optical receiver circuit over a single platform substrate. The conventional optical transmitter-receiver module has a silicon platform substrate 101. A silicon oxide film 102 overlies the silicon platform substrate 101. Interconnections 103 and 106 are selectively provided over the silicon oxide film 102 in an optical transmitter circuit region and an optical receiver circuit region respectively. A light receiving device 104 and a receiver LSI circuit 105 are further provided in the optical receiver circuit region. A light emitting device 107 and a transmitter LSI circuit 108 are further provided in the optical transmitter circuit region. Further, a shielding plate 109 is provided between the optical transmitter circuit region and the optical receiver circuit region. The above-described literature reported that the shielding plate reduces the crosstalk by about 20 dB at 1 GHz.
Both the light emitting device and the light receiving device are optically coupled through a ferrule to optical fibers. FIG. 2 is a plane view illustrative of the optical transmitter-receiver module of FIG. 1. The light receiving device 104 and the light emitting device 107 are optically coupled through a ferrule 114 to optical fibers 118 respectively. The ferrule 114 include short optical fibers 115 which are optically coupled to the light receiving device 104 and the light emitting device 107. The optical fibers 118 are further optically coupled to the short optical fibers 115 of the ferrule 114. The ferrule 114 may be made of a resin material. The ferrule 114 is aligned to the silicon platform substrate 101, so that the short optical fibers 115 which are aligned to the light receiving device 104 and the light emitting device 107 respectively.
The optical fibers 118 are supported by an optical connector 117 which is mechanically coupled with the ferrule 114, wherein the ferrule 114 has plural engaging projections 114a, whilst the optical connector 117 has plural engaging holes 117a which are engagable with the engaging projections 114a of the ferrule 114. This engagement mechanism aligns the optical connector 117 to the ferrule 114, whereby the optical fibers 118 are aligned to the short optical fibers 115.
The ferrule 114 is made of an optical shielding resin material which contains an light-absorbing additive such as a black pigment, in order to prevent that a stray light generated in the transmitter side undesirably enters into the receiver side. The interpose of the ferrule 114 between the optical connector and the transmitter circuit and the receiver circuit is disclosed in 2000 Electronics Information Communication Society S-3-140, entitled xe2x80x9cSM-Fiber MT-RJ Optical Transceiver Modulexe2x80x9d.
For integrally packaging the light emitting device and the light receiving device over a single substrate, silicon may often be selected for the substrate material because of its low cost and high beat conductivity. Silicon has a high heat conductivity of 150 W/mk whilst alumina has a high heat conductivity of 20 W/mk. Since silicon is relatively high in electrical conductivity as compared to insulators, as shown in FIG. 1, the light emitting device and the light receiving device are electrically coupled through the silicon platform substrate 101 but weakly, however, a relatively large cross talk appears through the silicon platform substrate 101 between the transmitter circuit and the receiver circuit.
Silicon is much lower in electrical conductivity than metal materials. Silicon has a specific resistivity of about 1E4 ohms cm, whilst copper has a specific resistivity of about 1.6E-64 ohms cm. Even if the silicon platform substrate 101 is grounded, the electrical coupling is still present between the transmitter circuit and the receiver circuit through the silicon platform substrate 101. It was confirmed that it is difficult to reduce the crosstalk to about xe2x88x9280 dB at 10 GHz.
It was proposed that in order to reduce the crosstalk, the silicon substrate is divided into the transmitter side and the receiver side for preventing the electrical coupling between the transmitter circuit and the receiver circuit through the silicon substrate. Separate packaging processes of the light emitting device and the light receiving device over the divided silicon substrate and separate alignment processes in optical axis are necessary. This increases the fabrication processes and also the final product cost.
As described above, the ferrule 114 is made of the optical shielding resin material which shields the stray light but does not shield electromagnetic waves. The resin ferrule 114 undesirably allows formation of an electromagnetic wave propagation route 116 at a confronting edge of the silicon platform substrate 101 to the ferrule 114. This electromagnetic wave propagation route 116 allows propagation of electromagnetic wave from the transmitter side to the receiver side, resulting in a possible generation of the undesirable crosstalk between the transmitter circuit and the receiver circuit.
If the ferrule 114 is made of a metal which is capable of shielding the electromagnetic wave for suppressing any formation of the electromagnetic wave propagation route 116, then the following difficulty is alternatively raised. As described above, the ferrule 114 has holes for incorporating the short optical fibers 115, wherein the holes have a diameter which is slightly larger than a diameter of the short optical fibers 115, and further the holes are distanced at a pitch exactly identical with a pitch between the light emitting device and the light receiving device. This means it necessary that the holes arc formed in the metal ferrule 114 at extremely high accuracy. The processings to the metal ferrule 114 at such extremely high accuracy is difficult, resulting in reduction of productivity.
In the above circumstances, the development of a novel free from the above problems is desirable.
Accordingly, it is an object of the present invention to provide a novel optical transmitter-receiver module free from the above problems.
It is a further object of the present invention to provide a novel optical transmitter-receiver module using a single electrically conductive platform substrate, which suppresses any formation of electrical crosstalk routes.
It is a still further object of the present invention to provide a novel optical transmitter-receiver module using a single electrically conductive platform substrate, which suppresses any formation of electromagnetic wave crosstalk routes and optical crosstalk route on a confronting side of the platform substrate.
The present invention provides an optical transmitter-receiver module comprising: an electrically conductive platform substrate having a transmitter region and a receiver region; a first insulating film extending over the transmitter region and the receiver region of the electrically conductive platform substrate; a first electrically conductive layer having a first fixed-potential, and the first electrically conductive layer extending in the receiver region and over the first insulating film; a second insulating film selectively extending over the first electrically conductive layer; an optical receiver circuit including at least a light-receiving device, and the optical receiver circuit existing over the second insulating film; an optical transmitter circuit including at least a light-emitting device, the optical transmitter circuit existing in the transmitter region, and the light-emitting device existing on the first insulating film; and a first electrically conductive shielding member spatially isolating the optical receiver circuit from the optical transmitter circuit, and the first electrically conductive shielding member being electrically coupled to the first electrically conductive layer, so that the first electrically conductive shielding member has the first fixed-potential.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.